Can we measure the brightness intensity of a Model(Cube)
A few alternatives:
Use the false colour view transform in the color management.


(from https://sobotka.github.io/filmic-blender/)

False Colour.
This Look is an extremely useful tool for evaluating your image in terms of the dynamic range and latitude. It is a colour coded “heat map” of your image values, according to the following codes:


Or render an image, and you can probe the values for the pixels under the cursor by pressing the right mouse button wile hovering over the image.

The values appear at the bottom of the image editor screen. You can see the coordinates of the pixel, and values before and after the color transforms of the color management.
Or you can measure the brightness of the image using a waveform monitor in the scopes section.
The scale here is 0 for black and 100% for white.

Note that these values are after the color transforms, or display referred values.